3 Answers
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This is a very subjective question because this is based on which plugins you are using. Generally I think there are more plugins available for 2.9.x or even 2.8.x but these numbers might be just misleading.

If you have not modified your blog at large so far and your theme and plugins are compatible with wordpress 3.0, you can consider upgrading. Do a backup first so you can go back in case something breaks.

It's good practice to be running the latest version, as it is more secure than previous ones.

Speaking specifically to 3.0, you would gain the ability to create your own custom types, right in the core, without having to hack it on with categories, tags, or plugins. There's also better menu management, and a new standard theme.

Not to mention, 3.1 is near release, and will include some new standard content types out of the box, so there's less need to create your own custom types. The custom type functionality is still in there, though, if you have specific needs.

Speaking to plugins, there tends to be better support for older WP versions, as they have been available for some time. Though in the past, WP updates have "broken" some plugins due to API or database changes. This has kept some users back from upgrading.